The present invention relates to a tubular steel product, a method of making such a tubular product, and to a gas generator housing based on such a tubular product.
The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention.
Openings can be formed in a tube wall of a tubular product through a drilling process. A downside of this approach is that the drilling process causes contamination, requires additional steps to remove burrs, and adds costs for checking the finished product. Another approach to form openings in a tube wall of a tubular product involves shear cutting. A downside of shear cutting of tubes of high-strength steel is the formation of microcracks around the hole. Tests have shown that most of those microcracks have a length of more than 50 μm. In addition, this approach results in burr formation with burr lengths of more than 40 μm. The extent of burr formation depends greatly on material properties, cutting speed, and cutting geometry.
Moreover, the finished opening becomes considerably wider in the fracture area. Typically the encountered widening of the opening amounts to more than 10% of the wall thickness.
Shear cutting has also the additional effect of increased strain hardening, causing hardening of more than 15% of the base hardness in the cutting area and fracture area. Tests have also shown an edge hardening up to a depth of about 100 μm.
There are applications, in particular when gas generators for airbag systems are involved, that are subject to an extreme pressure rise, e.g. when the airbag is deployed. When the rate of the pressure increase is very high, imperfections are caused in the microstructure, e.g. in the edge region of openings, and can lead to failure of the gas generator housing. The opening in the edge region of gas generator housings is hereby used to charge the gas generator. Subsequently, the opening is sealed by a plug using a welding process.
When the pressure rises at such rates, microcracks in the walls around the openings or changes in the microstructure, such as e.g. increase in hardness or strain hardening in the edge region of the opening, can induce stress peaks that may cause failure of the gas generator housing.
It would therefore be desirable and advantageous to address prior art problems and to obviate other prior art shortcomings.